Voltage reference circuits and biasing circuits of one form or another exist in many integrated circuits, and are especially common in large integrated circuit applications such as memories. In order to supply stable biasing and reference voltages, the power supply voltage must be stable and consistent throughout the integrated circuit memory. The power supply voltage is usually distributed across the integrated circuit by metal power supply lines, or conductors. As the length of these relatively thin metal lines increases, the resistance and capacitance between the connection point and the outside supply increases. The amount of voltage drop depends on the amount of current flowing through the conductor and the resistance of the conductor. In an integrated circuit memory the amount of current flow varies, causing voltage fluctuations at the demand end of the conductor. Normally, the voltage drop along a power supply conductor is reduced by making the metal lines of the power supply conductors as wide as possible. But there is a tradeoff between the width of the metal lines and integrated circuit size. Size constraints do not normally allow the metal lines to be wide enough to reduce the series resistance and capacitance enough to provide a power supply voltage adequate to guarantee proper operation of the integrated circuit at very high speeds.
The power supply lines can also be a source of noise on an integrated circuit memory. The noise immunity of an integrated circuit is commonly specified in terms of noise margins. Noise margins are usually given for worst case conditions, which includes the most unfavorable connections of the input terminals and the poorest combination of circuit and device parameters, along with a maximum fan-out. As the operating speed of an integrated circuit memory increases, the negative effects of noise or a voltage drop in the power supply increase. The more noise or voltage drop there is on the power supply lines, the smaller the range of variability allowed in the reference and bias voltages because the memory must operate within narrower margins.
The introduction of the so called revolutionary pinout technique has alleviated some of the problems with power distribution on integrated circuit memories by decreasing the power supply conductor lengths and increasing the number of power supply bonding pads and power supply pins. But increasing the size and density of the integrated circuit memory have correspondingly caused an increase in the length of the power supply conductors. Nevertheless, as higher speeds are demanded of integrated circuit memories, any power supply variations that interfere with system operation become intolerable. The biasing and reference voltage generation circuits must remain stable and must supply constant voltage levels in order to insure accurate system operation.